1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet effectively applicable to motors and actuators of various types, and more particularly relates to a method for producing an iron-based rare earth alloy magnet including multiple ferromagnetic phases.
2. Description of the Related Art
Recently, it has become more and more necessary to further enhance the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot show that high remanence Br of 0.5 T or more.
An Sm—Co magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet with that high remanence Br of 0.5 T or more. Examples of other high-remanence magnets include Nd—Fe—B type magnets produced by a powder metallurgical or melt quenching process. An Nd—Fe—B type magnet of the former type is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Nd—Fe—B type magnet of the latter type is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance.
However, the Sm—Co magnet is expensive, because Sm and Co are both expensive materials.
As for the Nd—Fe—B type magnet on the other hand, the magnet is mainly composed of relatively inexpensive Fe (typically accounting for 60 wt % to 70 wt % of the total quantity), and is much less expensive than the Sm—Co magnet. Nevertheless, it is still expensive to produce the Nd—Fe—B type magnet. This is partly because huge equipment and a great number of process steps are needed to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for 10 at % to 15 at % of the total quantity. Also, a powder metallurgical process normally requires a relatively large number of process steps by its nature.
Compared to an Nd—Fe—B type sintered magnet formed by a powder metallurgical process, an Nd—Fe—B type rapidly solidified magnet can be produced at a lower process cost by a melt quenching process. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
For these reasons, an Nd—Fe—B type rapidly solidified magnet produced by a melt quenching process has a remanence Br lower than that of a magnetically anisotropic Nd—Fe—B type sintered magnet produced by a powder metallurgical process.
As disclosed in Japanese Laid-Open Publication No. 1-7502, a technique of adding at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr in combination effectively improves the magnetic properties of an Nd—Fe—B type rapidly solidified magnet. When these elements are added, the magnet can have its coercivity HcJ and anticorrosiveness increased. However, the only known effective technique of improving the remanence Br is increasing the density of a bonded magnet.
As for an Nd—Fe—B type magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1998, pp. 669-670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd3.8Fe77.2B19, where the subscripts are indicated in atomic percentages) and an Fe3B primary phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe3B and hard magnetic Nd2Fe14B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are dispersed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a “nanocomposite magnet”. It was reported that a nanocomposite magnet like this has a remanence Br of as high as 1 T or more. But the coercivity HE thereof is relatively low, i.e., in the range from 160 kA/m to 240 kA/m. Accordingly, this permanent magnet is applicable only when the operating point of the magnet is 1 or more.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, Japanese Laid-Open Publication No. 3-261104, U.S. Pat. No. 4,836,868, Japanese Laid-Open Publication No. 7-122412, PCT International Publication No. WO 00/03403 and W. C. Chan et. al., “The Effects of Refractory Metals on the Magnetic Properties of α-Fe/R2Fe14B-type Nanocomposites”, IEEE Trans. Magn. No.5, INTER-MAG. 99, Kyongiu, Korea, pp.3265-3267, 1999. However, none of these proposed techniques can always obtain a sufficient “characteristic value per cost”.